1. Field of the Invention
This invention relates generally to adhesive compositions containing adhesion promotor compounds for promoting adhesion of such compositions to metal substrates.
2. Description of the Related Art
In the field of adhesive bonding, a wide variety of adhesive compositions are employed for bonding metal substrates to one another and to other substrate materials. Examples of such adhesive compositions include anaerobic curing adhesives, thiolene compositions, and tough acrylic formulations.
Numerous peroxy-type initiators have been employed with various curable components containing ethylenically unsaturated polymerizable and/or crosslinkable moieties in such adhesive compositions, to initiate or assist in curing thereof.
In compositions in which peroxy-type initiators have been employed, a preferred peroxy-type initiator which has come into widespread usage is t-butyl perbenzoate. The adhesive compositions which have utilized t-butyl perbenzoate and related perbenzoate initiators variously include: anaerobic curing compositions, such as those disclosed in U.S. Pat. No. 3,046,262 to V. K. Krieble; polymerizable compositions based on unsaturated curable poly(alkylene) ether polyol-based resins, as described in U.S. Pat. No. 4,309,526 to L. J. Baccei; and so-called "tough acrylic" formulations comprising acrylic/rubber copolymers capable of being cross-linked through acryloxy and ethylenically unsaturated rubber functional groups, e.g., the tough acrylic composition commercially available under the trademark Depend.RTM. (Loctite Corporation, Newington, Connecticut).
In numerous adhesive bonding applications, the aforementioned adhesive compositions utilizing t-butyl perbenzoate are employed for bonding of substrates comprising metal surfaces. In such applications, metal cations on the substrate in conjunction with other accelerators may cause the t-butyl perbenzoate to spontaneously decompose to form benzoic acid and t-butanol. The resulting product benzoic acid, which may be present in concentrations as high as 2-3 percent by weight of the adhesive composition, can then react with the metal cations to form metal-complexed benzoic acid salts.
The benzoic acid and butanol reaction products of the decomposition of the t-butyl perbenzoate initiator are detrimental to the cohesive strength of the adhesive composition and to its adhesion to the substrate. These reaction products function as diluents which do not promote the intended polymerization and/or crosslinking of the ethylenically unsaturated moieties in the composition. Further, the salts of benzoic acid formed at the substrate surface/adhesive composition interface prevent the adhesive composition from achieving good adhesion to the metal surface.
Accordingly, it would be a significant advance in the art to provide a peroxy initiator for use in adhesive compositions such as those illustratively described above, wherein the initiator decomposition products are copolymerizable with the adhesive composition and form copolymerizable metals salts at the interface.
It is therefore an object of the invention to provide a peroxy initiator of such type, which is usefully employed in adhesive compositions to effect polymerization and/or crosslinking thereof, in a manner yielding good adhesion and cohesive strength characteristics for the cured adhesive composition.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.